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Nevis Laboratories Summer 2000 Education Workshop on “Electron Nevis Laboratories Summer 2000 Education Workshop on “Electron Bubble Particle Detector R&D” Text and design: Jeremy Dodd Cover Art: Jean Therrien “Cryogenic Bubbles”, reproduced by permission of the photographer. Nevis Laboratories Summer 2000 Education Workshop on “Electron-Bubble Particle Detector R&D” Overview During summer 2000, Columbia University’s Nevis Laboratories hosted a two-month Workshop to study a new detector, using cryogenic liquid as the detecting medium, which could provide a compact and efficient solution for a next-generation neutrino detector. Possible applications of this technology for detectors at future colliding-beam facilities are also being noted. Education and training were a major emphasis of the Workshop, with a team of ten high school stu- dents, undergraduates and high school teachers working alongside Nevis scientists and technicians to determine the critical physics and technology issues, and to develop a conceptual design for the detec- tor. The two high school teachers are part of the QuarkNet program, and in addition to their research activities during the summer, they also developed curriculum material for use in the classroom, and made plans for a QuarkNet Associate Teacher Institute at Nevis next year. By the end of the Workshop, the team had participated in the design of a small cryogenic test facility that will be built over the winter, fixed the conceptual design of a liquid helium solar neutrino detector, completed an experimental program of measurements of avalanche behavior in unquenched noble gases, and attended a comprehensive series of lectures on fundamental physics and detectors. The students and teachers were exposed to most aspects of the science research experience, from initial reading and literature searches, through experiment design and construction, data-taking and analysis, and finally in making presentations and writing reports for an audience of peers. The students’ appreciation and enjoyment of their summer research work was such that, in addition to our two returning QuarkNet teachers, three high school students and two undergraduates have already expressed a desire to continue working with us next summer. We look forward to building upon this year’s success with a Second Nevis Workshop in summer 2001! Outreach Activities at Nevis Nevis has had a summer program for undergraduate physics students since its origin. Most of them have been students at Columbia. Columbia undergraduates have also taken part in research during the academic year. Starting in 1997, we have agreed to mentor high school students in research projects of their own. In order to launch this program, we generated a “packet” of information about our labora- tory, particle physics, detectors and accelerators, and Women in Physics, and distributed it to more than one hundred high school science teachers in our region. We have fielded many questions from teachers and students over the years that followed. In many cases, we have helped students find mentors in institutions closer to their homes than Nevis, or in research areas that correspond more nearly to their interests. We interview some of the students who seem a promising match to our possibilities, and have established long-term projects with a number of students. Some of them have learned modern physics in tutoring sessions with us, and then moved on to research mentors at the Columbia campus or at other institutions in our region, while others have moved into research projects at Nevis. The students begin by coming to Nevis during the school year every week or two for a tutoring session, where we go over elementary Modern Physics at the level of a non-calculus college course. Although most of the stu- dents have not taken any high school physics, we find that the material of Modern Physics, Relativity and Wave Mechanics, does not substantially depend on the material taught in the high school physics courses. The students study for a week or so, and come back with questions on the previous session, and start on the next stage. For our highly motivated students, this has been quite successful. Some of the students have completed research projects at Nevis with outstanding results, including two students who have won top prizes for their work. Max Lipyanskiy was an Intel Science Talent Search Semifinalist in 1999, while Mike Lowinger won First Prize in the Communications and Electronics Association Competition 2000. In the past three summers we started to function as an informal team, with high school students work- ing with undergraduates and Nevis staff, and in one case with a high school teacher, and we found that combination very effective. The undergraduates have unique advantages as part of the mentoring team. This led us to set up our more formal Workshop this summer. We thought that a common focus on an ultimate goal would tie together the research of a number of students. We also wanted to choose a new project, so the students could see how a scientific collaboration gets started. Tying in nicely with the new Nevis initiative in cryogenic detector development, this led to the choice of the electron-bubble (“eBubble”) detector as the focal point for the summer 2000 Workshop. Many of the seminars during the Workshop had a direct connection with this project. The Physics The focus of the Summer Workshop was the investigation of a novel detector technology, which may have applications in future experiments to address critical questions in particle physics, astrophysics and cosmology. In particular, we are interested in the possibility of using cryogenic detectors deep underground to detect the interactions of neutrinos, from both natural and man-made sources. The Sun, as viewed by the SOHO Extreme ultraviolet Imaging Telescope For more than three decades, underground experiments measuring the flux of neutrinos from the Sun have found a significant deficit in the observed rate compared to the predictions of the Standard Solar Model. A possible explanation for this discrepancy is that (electron) neutrinos produced in the core of the Sun are modified along their path to Earth, transforming into another neutrino species via neutrino oscillations. Current experiments, relying on radiochemical techniques in gallium or chlorine, or on the production of Cerenkov radiation in water-based detectors, are mostly sensitive to the upper range of the solar neutrino energy spectrum, of order 1–10 MeV. However, this window encompasses only a small fraction of the total neutrino flux, about 98% of which is expected to have energies less than 1 MeV. Moreover, of the published experiments, only the Cerenkov detectors provide information about the energies of the incoming neutrinos. The next major goal of solar neutrino astronomy is to measure neutrino fluxes in this low energy region, and in particular to measure the flux from the dominant pp reaction, which peaks in the range of 200–300 keV. It would be very desirable to measure also the energy spectrum of the scattered electrons produced by neutrino interactions. These measurements will provide a simultaneous and critical test of stellar evolution theory and of neutrino oscillation solutions. The detection techniques we are investigating in the Nevis Workshop may have important applications in this domain. The solar neturino spectrum (from J.N. Bahcall et al) There is now strong evidence for neutrino oscillations from underground experiments. In addition to very long baseline measurements of low-energy neutrinos from the Sun, complementary regions of the oscillation parameter space can be explored using reactor and accelerator sources of neutrinos. Accel- erator sources of muons are particularly attractive in this respect since the muon decay gives rise to both electron and muon neutrinos with known energy spectra, which can be directed to a distant experi- ment. These experiments will lead to new challenges for detectors, most notably in the requirement of a fine-grained structure over very large volumes, and we may anticipate that a technology based on the concept of slowly drifting electrons in cryogenic media will play an important role here also. The features required of a detector that could measure neutrino interactions in this regime are: good spatial resolution at very low energies, in a large volume with the lowest possible background rates. The electron-bubble detector under study in the Nevis Workshop may provide a solution to these re- quirements. The Electron-Bubble Detector The physics arguments outlined above for measuring neutrino interactions give rise to the following requirements for an ideal detector: • Substantial mass, varying for the different physics goals • Sensitivity to “small” particle energies, therefore good energy resolution • Good spatial resolution, to detect short-range tracks • Particle identification, to distinguish e, p, n, γ • Capability to measure sign and momentum, of charged particles • Track imaging, in a large active volume • Read out of large number of volume elements, at a reasonable data rate • Underground site, to shield cosmic-ray backgrounds The requirement of good spatial resolution is most easily met by working in a low- or moderate-density detection medium, with minimum diffusion. This implies that we will achieve better performance at low temperatures. The requirement of charged particle momentum measurement, via curvature in a magnetic field, suggests that a “light” (low-Z) material is preferred. Together, these constraints lead us to consider the detection of ionization tracks in liquids or solids at low temperature. In order to deal with the large number of individual three-dimensional volume elements (voxels), a high degree of serialization and multiplexing must be incorporated in the readout. Drifting the signals through one dimension of the detector volume on a timescale of seconds or longer would therefore be desirable. Three cryogenic liquids, liquid helium, liquid hydrogen and liquid neon, have physical properties which lend themselves to meeting the properties of the ideal detector. The three liquids share a unique mecha- nism of electron transport.
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